Non-hazardous water-based polyurethane dispersion

ABSTRACT

A water-based polyurethane dispersion including a chain-extended polyurethane, characterized in that the polyurethane dispersion includes 2,5,7,10-tetraoxaundecane. The water-based polyurethane dispersion is suitable as a non-hazardous water-based coating composition for coatings with a particularly good adhesion on plastic or rubber substrates. The solvent 2,5,7,10-tetraoxaundecane contained therein imparts good film coalescing, good adhesive properties and high strength of the polyurethane dispersion.

TECHNICAL FIELD

The invention relates to water-based polyurethane dispersions and theiruse for coatings on plastic or rubber substrates.

BACKGROUND OF THE INVENTION

Water-based polyurethanes dispersions (PUDs) are one of the fastestgrowing fields in the coating industry due to their technologicaladvances that has made them an effective substitute for thecorresponding solvent based coatings.

PUDs are used in many industrial and commercial applications.

Water-based polyurethane dispersions usually include solvents which areoften necessary for formulating PUDs and/or preparing precursorsthereof. There are countless commercially available water-basedpolyurethane dispersions containing pyrrolidone solvents. These havebeen industry standard for many years.

To move away from pyrrolidone solvents, the same results could beachieved by using different solvents. Typical examples of other solventsinclude glycol ethers and dimethylsulphoxide (DMSO). PUDs can also beprepared solvent free if the dispersed polymer is soft enough tocoalesce on its own. There is however only a limited range ofapplications for solvent free PUDs.

Many attempts have been made to provide alternative solvents. The use ofalternative solvents has typically failed because the solvation power ofthe alternative solvents is not good enough. They either do not dissolvethe required components sufficiently or require such large amounts ofsolvent, that the volatile organic content (VOC) of the finished productis unacceptably high. Other solvents may also be hazardous to varyingdegrees. These hazards carry through to the final product and result inhazard classifications that are unwanted.

Moreover, polyurethane dispersions including the conventional solventsused are often not satisfactory with respect to the substrate adhesionwhen they are used as coatings for plastic or rubber substrates.

Solvent free PUDs have a limited range of applications because thepolymer must coalesce on its own. At ambient temperatures this meansusing soft polymers that would not be suitable for hard wearing or toughapplications. If a rigid polymer was prepared solvent free, it wouldrequire heated drying to achieve film formation. This is an energy andlabour intensive process that not all users of PUDs have access to.

US 2013/0316098 A1 relates to an aqueous cationic polyurethanedispersion comprising an aqueous dispersion of polyurethane havingtethered tertiary amino groups separated from backbone by at least twointervening atoms or terminal tertiary amino groups with multipletertiary amino groups per terminus. Solvents may be used but are notpreferred. A number of examples for solvents are mentioned, inter aliaethylene glycol monomethyl ether formal.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention was to provide polyurethanedispersions which can overcome the drawbacks of the prior art discussedabove. In particular, an object was to provide polyurethane dispersionswith good film forming properties and substrate adhesion, especiallywhen used for coating plastic or rubber substrates.

Moreover, hazardous solvents and high amount of solvents should beavoided in order to obtain a more environmentally friendly product. Inaddition, it should be possible to prepare the polyurethane dispersionby a method of manufacture under convenient conditions.

Surprisingly, it was found that the above object could be solved byusing 2,5,7,10-tetraoxaundecane (TOU) as a solvent in polyurethanedispersions, especially for use in coatings for plastics or rubber.

Accordingly, the invention is related to a water-based polyurethanedispersion comprising a chain-extended polyurethane, wherein thepolyurethane dispersion comprises 2,5,7,10-tetraoxaundecane. In atypical embodiment, the chain-extended polyurethane has carboxylategroups.

In particular, the invention refers to the use of2,5,7,10-tetraoxaundecane (TOU) as a coalescent in polyurethanedispersions, particularly for use in plastics or rubber coatings.

TOU as a coalescent is surprisingly advantageous in several purposes.Firstly, it can act as a solvent and/or diluent in the preparation ofthe polyurethane dispersion. Moreover, it serves as an efficient filmcoalescing aid during curing of the dispersion, thus improving the filmforming properties. And finally, TOU improves the adhesion to plastic orrubber substrates. It brings benefits to film adhesion and promotes astronger bond between the substrate and dried PUD film. Thus, anon-hazardous water-based coating composition can be provided for use onplastic or rubber substrates.

As to the application on plastic or rubber substrates, the inventive PUDcan modify and/or improve the surface of plastic or rubber substrates tosuit customers requirements. These requirements can be very varied asexplained previously.

Surprisingly, the PUD containing TOU shows not only good film formingand adhesion properties, but also a very high tensile strength with goodelongation. Its tensile strength is markably higher than that of aconventional PUD containing N-methyl-2-pyrrolidone (NMP) orN-ethyl-2-pyrrolidone as solvent (NEP) instead of TOU.

The invention is also related to a process for producing thepolyurethane dispersion, a coating composition comprising thepolyurethane dispersion and the use of TOU as coalescent for PUD,particularly rigid PUDs. Preferred embodiments of the invention aredescribed in the dependent claims.

DETAILED DESCRIPTION OF THE INVENTION

Substance names beginning with “poly”, such as e.g. polyol,polyisocyanate or polyurethane, designate substances which formallycontain, per molecule, two or more of the functional groups occurring intheir names. A polyol is e.g. a substance having two or more hydroxylgroups.

An isocyanate-terminated polymer is a polymer or prepolymer having atleast one terminal isocyanate group, in particular two terminalisocyanate groups. The term prepolymer here generally refers tooligomers or polymers which are used as intermediate products forproducing polymers with higher molecular weight.

Chain extenders are difunctional compounds, i.e. compounds having twofunctional groups which are reactive with functional groups, usuallyterminal functional groups, of a prepolymer so that the chain extendercan react with the prepolymer to form a chain-extended polymer.

The average molecular weight is understood to mean the number averagemolecular weight, as determined using conventional methods, preferablyby gel permeation-chromatography (GPC) using polystyrene as standard,styrene-divinylbenzene gel with porosity of 100 Angstrom, 1000 Angstromand 10000 Angstrom as the column and tetrahydrofurane as a solvent, at35° C.

In the following the invention is described with respect to the PUDproduct and the method for producing it, respectively. Details given forthe product of course equally apply to the method and vice versa, e.g.with respect to the starting materials and their proportions, whereapplicable.

The invention relates to a water-based polyurethane dispersioncomprising a chain-extended polyurethane, wherein the polyurethanedispersion comprises 2,5,7,10-tetraoxaundecane.

The solvent 2,5,7,10-tetraoxaundecane, abbreviated TOU, is a well-knowncompound and can be prepared by the skilled person according to knownsynthesis procedures. TOU is commercially available from a number ofsuppliers. It can act as a solvent and/or diluent and/or coalescent inthe water based PUD.

A particular surprising benefit of TOU in the PUD is that TOU acts as anadhesion promoter to plastic or rubber substrates when the inventive PUDis used for coating plastic or rubber substrates.

Another surprising benefit of TOU in the PUD is that the cured PUD has avery high strength, markably higher than a corresponding PUD with NMP orNEP instead of TOU.

Water-based polyurethane dispersions are aqueous dispersions ofpolyurethane particles. The polyurethane particles are stabilizedionically or non-ionically. Accordingly, PUDs have been classified intothree categories, namely anionic, cationic and non-ionic PUDs, whereinthe PUD is stabilized by anionic groups, cationic groups or hydrophilicunits, respectively, contained in the polyurethane. Hence, thewater-based polyurethane dispersion of the invention can be an anionic,cationic or non-ionic water-based polyurethane dispersion.

Anionic PUDs are usually stable at alkaline pH values >7. Cationic PUDsare usually stable at acid pH<7 and normally based upon alkylated orprotonated tertiary amines. Non-ionic PUDs are non-ionisable and oftenstable over a very wide pH range.

The chain-extended polyurethane of the water-based polyurethanedispersion may have ionic groups or hydrophilic non-ionic groups. Thechain-extended polyurethane of the water-based polyurethane dispersionmay have anionic groups such as carboxylate groups or sulfonate groups,cationic groups such as ammonium cations, e.g. alkylated or protonatedsecondary or tertiary amines, or non-ionic hydrophilic units such aspolyethylene oxide units. The non-ionic hydrophilic units may becontained in the backbone chain of the polyurethane or are preferablycontained in pendant or terminal groups of the polyurethane. The ionicgroups, in particular carboxylate groups, may be pendant groups orcontained in pendant side groups of the polyurethane.

It is preferred that the chain-extended polyurethane has anionic groupssuch as carboxylate groups or sulfonate groups, in particularcarboxylate groups.

The counter ion of the anionic groups, such as carboxylate groups, maybe any conventional counter ion, e.g. alkali metal ions, alkaline earthmetal ions, NH₄ ⁺ or organic ammonium ions. Organic ammonium ions havingone or more organic groups, in particular alkyl groups, are preferred.The counter ion of the cationic groups, such as ammonium groups, may beany conventional counter ion, e.g. OH⁻, halogenid anions or complexanions such as BF₄ ⁻. Particularly preferred is trimethyl ammonium ortriethyl ammonium.

The water-based polyurethane dispersion may comprise e.g. 1 to 20% byweight, preferably 5 to 15% by weight, of 2,5,7,10-tetraoxaundecane,based on the total weight of the water-based polyurethane dispersion.

The water-based polyurethane dispersion may comprise e.g. 3 to 60% byweight, preferably 10 to 40% by weight, of 2,5,7,10-tetraoxaundecane,based on the total weight of the chain-extended polyurethane in thewater-based polyurethane dispersion.

The water-based polyurethane dispersion may comprise e.g. 45 to 80% byweight, preferably 50 to 75% by weight, of water, based on the totalweight of the water-based polyurethane dispersion.

The water-based polyurethane dispersion may comprise e.g. 20 to 55% byweight, preferably 25 to 50% by weight, of the chain-extendedpolyurethane, preferably the chain-extended polyurethane havingcarboxylate groups, based on the total weight of the water-basedpolyurethane dispersion.

The water-based polyurethane dispersion may optionally comprise one ormore additives, which are common for PUDs, in particular for PUDs usedas coating compositions. Examples of suitable additives are defoamers,surfactants, rheological additives, thickening agents or organicsolvents, which are different from TOU, such as e.g. butyl diglycol orisopropyl alcohol. Suitable commercial products of such additives aree.g. Byk® 012, Byk® 024, Byk® 348, Surfynol® MD20, Rheovis® PU1214 orBorchi Gel® CW44.

The chain-extended polyurethane in the PUD is preferably the reactionproduct of an isocyanate-terminated polyurethane prepolymer and at leastone chain extender, which is preferably a polyamine, wherein theisocyanate-terminated polyurethane prepolymer preferably has ionicgroups, in particular carboxylate groups.

In the chain-extended polyurethane, the acid groups or amine groups, ifpresent, are completely or partially converted into ionic groups with abase or an acid. Suitable starting materials are described in thefollowing.

The isocyanate-terminated polyurethane prepolymer is preferably areaction product of

-   -   at least one first polyol,    -   at least one second polyol, which is different from the first        polyol, and    -   at least one diisocyanate,        wherein the at least one second polyol is preferably selected        from a polyether polyol having ethylene oxide units, a polyol        having an acid group, preferably a carboxyl group, and a polyol        having an amine group.

Most preferred is a second polyol having a carboxylate group.

The at least one first polyol may be one or more polyols. There is ahuge variety in the potential polyols. The polyol may have two or morehydroxyl groups, e.g. a diol or a triol or a mixture thereof. The atleast one polyol is however preferably one or more diols, e.g. one diolor a mixture of two or more diols.

Examples of suitable first polyols are polyoxyalkylenepolyols, alsoreferred to as polyetherpolyols, polyesterpolyols, polycarbonatepolyols,poly(meth)acrylate polyols, polyhydrocarbon-polyols,polyhydroxy-functional acrylonitrile/butadiene copolymers and mixturesthereof, wherein said polyols are preferably diols.

Examples of polyetherpolyols are polyoxyethylenepolyols,polyoxypropylenepolyols and polyoxybutylenepolyols, in particularpolyoxyethylenediols, polyoxypropylenediols, polyoxybutylenediols,polyoxyethylenetriols and polyoxypropylenetriols. Further examples ofpolyetherpolyols are so-called ethylene oxide-terminated(“EO-endcapped”, ethylene oxide-end-capped) polyoxypropylenepolyols, orstyrene-acrylonitrile-grafted polyetherpolyols.

Examples of polyesterpolyols are polyesterdiols obtained from thereaction of difunctional alcohols, such as 1,2-ethanediol, diethyleneglycol, 1,2-propanediol, dipropylene glycol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,4-cyclohexane dimethanol orneopentylglycol, with dicarbonic acids or anhydrides or esters thereof,such as succinic acid, glutaric acid, adipinic acid, suberic acid,sebacinic acid, dodecane dicarbonic acid, maleinic acid, fumaric acid,phthalic acid, isophthalic acid, terephthalic acid or hexahydroterephthalic acid, or polyesterdiols obtained from the polymerization oflactones, particularly ε-caprolactone.

Examples of polycarbonatepolyols are polycarbonatepolyols obtained fromthe reaction of difunctional alcohols, such as 1,2-ethanediol,diethylene glycol, 1,2-propanediol, dipropylene glycol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,4-cyclohexane dimethanol orneopentylglycol or mixtures thereof, with dialkyl carbonates or diarylcarbonates oder phosgene.

Further examples of polyols to be used in the present invention arepolyhydroxy-functional fats and oils, for example natural fats and oils,or polyols obtained by chemical modification of natural fats and oils,so-called oleochemical polyols.

The first polyol contains preferably at least one polyester polyol orpolycarbonate polyol. Such polyols enable rigid PUDs with high strength.

The first polyol preferably has an average molecular weight of from 400to 10'000 g/mol, preferably from 500 to 8'000 g/mol, more preferablyfrom 500 to 4'000 g/mol, most preferably from 500 to 2'000 g/mol, and/oran average OH functionality in the range from 1.6 to 2.5, preferablyfrom 1.8 to 2.0.

A particularly preferred polyol to be used as the first polyol is apolycarbonate diol, preferably with an average molecular weight of from500 to 2'000 g/mol. Such polyols are commercially available. Suchpolyols enable rigid PUDs with high strength

The at least one second polyol is different from the at least one firstpolyol. It is preferably selected from a polyether polyol havingethylene oxide units, a polyof having an acid group, preferably acarboxyl group, and a polyol having an amine group.

Examples of a polyether having ethylene oxide units are polyetherpolyolshaving ethylene oxide units such as polyethylene glycol, or polyethyleneglycol dialkylethers or polyethylene glycol monoalkylethers, wherein twoprimary hydroxyl groups are incorporated in the polymer. A preferredpolyether having ethylene oxide units may be a difunctional polyethyleneglycol monomethyl ether, wherein two primary hydroxyl groups areincorporated in the polymer such as Ymer® N120 (from Perstorp).

The polyol having an acid group is preferably a diol having a carbonicor sulfonic acid group, preferably a carboxyl group. Particularlypreferred is dimethylol propionic acid (DMPA) or dimethylol butanoicacid (DMBA).

The polyol having an amine group has preferably one or two, particularlyone, tertiary amine group. It is preferably a diol. Particularlypreferred is N-alkyl-diethanolamine, wherein alkyl is preferablyC₁-C₄-alkyl such as ethyl or methyl. N-methyl-diethanolamine isparticularly preferred.

Particularly preferred as second polyol is a polyol having a carboxylgroup, such as dimethylol propionic acid (DMPA) or dimethylol butanoicacid (DMBA). Most preferred is DMPA.

Low molecular weight di- or polyhydric alcohols, e.g. with a molecularweight of less than 250 g/mol, can be used together with the first andthe second polyol. Examples thereof are 1,2-ethanediol, 1,2- and1,3-propanediol, neopentylglycol, diethylene glycol, triethylene glycol,the isomeric dipropylene glycols and tripropylene glycols, the isomericbutanediols, pentanediols, hexanediols, heptanediols, octanediols,nonanediols, decanediols, undecanediols, 1,3- and1,4-cyclohexanedimethanol, hydrogenated bisphenol A, dimeric fattyalcohols, 1,1,1-trimethylolethane, 1,1,1-trimethylolpropane, glycerol,pentaerythritol, sugar alcohols, such as xylitol, sorbitol or mannitol,sugars, such as sucrose, other alcohols having a higher functionality,low molecular weight alkoxylation products of the abovementioned di- andpolyhydric alcohols, and mixtures thereof.

The at least one diisocyanate may be one or more diisocyanate. There isa huge variety in the potential diisocyanates. Suitable diisocyanatesare those which are amply known in polyurethane chemistry, orcombinations thereof. Suitable diisocyanates are aliphatic or aromaticdiisocyanates, wherein aliphatic diisocyanates are preferred. Aliphaticdiisocyanates include cycloaliphatic diisocyanates wherein at least oneisocyanate is bound to cycloaliphatic carbon atom. Cycloaliphaticdiisocyanates are particularly suitable.

Examples of suitable aliphatic diisocyanates, including cycloaliphaticdiisocyanates are 1,6-diisocyanatohexane (HDI),1-isocyanato-5-isocyanatomethyl-3,3,5-trimethylcyclohexane (IPDI),bis-(4-isocyanatocyclohexyl)methane (H₁₂ MDI),1,3-bis(1-isocyanato-1-methylethyl)benzene (m-TMXDI or TMXDI) and/ortechnical-grade isomer mixtures or derivatives thereof. Examples ofsuitable aromatic polyisocyanates are 2,4(6)-diisocyanatotoluene (TDI)and bis(4-isocyanatophenyl)methane (MDI).

Particularly preferred as diisocyanate is bis(4-isocyanatocyclohexyl)methane, isophorone diisocyanate or TMXDI.

Most preferred is bis(4-isocyanatocyclohexyl) methane. This diisocyanateenables rigid PUDs with high strength.

If the isocyanate-terminated polyurethane prepolymer contains carboxylgroups, these are at least partly neutralized with a suitable base andconverted into carboxylic anions. The neutralization typically takesplace after the reaction of the polyols with the diisocyanate iscompleted and before the isocyanate-terminated polyurethane prepolymeris mixed with water.

Examples of suitable bases for neutralization are tertiary amines suchas trimethylamine, triethylamine, triisopropylamineN,N-dimethylethanolamine, N-methyl-diethanolamine,N-methyldiisopropanolamine, dimethylisopropanolamine,N-methylmorpholine, N-ethylmorpholine or triethanolamine, or alkalimetal hydroxides such as lithium hydroxide, sodium hydroxide, orpotassium hydroxide or mixtures thereof.

Preferred thereof are trimethylamine, triethylamine, n-ethyl morpholineor methyl diethanolamine or mixtures thereof. Particularly preferred istrimethylamine or triethylamine.

If the isocyanate-terminated polyurethane prepolymer contains aminegroups, these are at least partly neutralized with a suitable acid andconverted into ammonium cations. The neutralization typically takesplace after the reaction of the polyols with the diisocyanate iscompleted and before the isocyanate-terminated polyurethane prepolymeris mixed with water.

Examples of suitable acids for neutralization are carboxylic acids suchas acetic acid, HCl or phosphoric acid.

Finally, the isocyanate-terminated polyurethane prepolymer is mixed withwater and chain extended with a suitable chain extender.

The chain extender may be selected from at least one of water, inorganicor organic polyamine having an average of about two or more primaryand/or secondary amine groups, amine functional polyoxyalkylenes, ureas,or their combinations, wherein polyamines are preferred. Particularlypreferred are diamines with two primary and/or secondary amine groups.Polyamines as used herein also refer to polyhydrazines, in particulardihydrazides. The polyamine is preferably a linear or branchedalkylenediamine, e.g. a linear or branched C₂₋₁₂-alkylenediamine, or acycloalkane or alkyl substituted cycloalkane having two amine groups.

Suitable examples of chain extenders include ethylene diamine (EDA),diethylenetriamine (DETA), 1,3-bisaminomethylbenzol (meta-xylenediamineor MXDA), aminoethylethanolamine (AEEA), urea or derivatives of urea,adipic dihydrazide, hexamethylenediamine (HAD), hydrazine,isophoronediamine (IPDA), 2-methylpentamethylenediamine (MPMD) or anydesired combinations of these chain extenders.

Preferred chain extenders are EDA, MPMD or HDA.

The invention further relates to a process for producing the water-basedpolyurethane dispersion as described above, the process comprising thesteps of

-   a) reacting at least one first polyol, at least one second polyol    and at least one diisocyanate in the presence of    2,5,7,10-tetraoxaundecane and optionally a catalyst to form an    isocyanate-terminated polyurethane prepolymer, wherein the second    polyol is different from the first polyol,-   b) optionally at least partly neutralizing the isocyanate-terminated    polyurethane prepolymer to obtain a neutralized    isocyanate-terminated polyurethane prepolymer, and-   c) dispersing the optionally neutralized prepolymer in water and    adding a chain extender, preferably a polyamine, to obtain the    water-based polyurethane dispersion comprising the chain-extended    polyurethane and 2,5,7,10-tetraoxaundecane.

Neutralization of the prepolymer (step b) is preferred for prepolymerscontaining a second polyol with acid or amine groups.

The starting materials for the process are described above. Theproportions of the starting materials may vary to a large extentdepending on the starting materials used and the desired properties ofthe prepolymer to be produced.

Preferably the at least one second polyol has carboxyl groups and isdimethylol propionic acid (DMPA) and/or the at least one diisocyanate isan aliphatic diisocyanate, preferably bis(4-isocyanatocyclohexyl)methane.

The base to neutralize the acid groups is preferably a tertiary amine.

The isocyanate-terminated prepolymer is preferably made from a mixturecomprising e.g. 20 to 70%, preferably 30 to 60%, by weight of one ormore first polyols, 1 to 20%, preferably 2.5 to 10%, by weight of one ormore second polyols and 10 to 60%, preferably 20 to 60%, by weight ofone or more diisocyanates, based on the total weight of first polyols,second polyols and diisocyanates.

This mixture contains preferably 3 to 60% by weight, particularly 10 to40% by weight, of 2,5,7,10-tetraoxaundecane based on the total weight offirst polyols, second polyols, diisocyanates and2,5,7,10-tetraoxaundecane.

In order to obtain an isocyanate-terminated polyurethane prepolymer thediisocyanate is used in excess, i.e. the molar ratio of isocyanategroups to hydroxyl groups (NCO/OH ratio) in the mixture to bepolymerized in step a) is greater than 1 and preferably in the range of1.5 to 2.5 more preferably 1.8 to 2.2.

The process steps a) to c) may be carried out under an inert gasatmosphere. In particular, it is preferred that first step a) is carriedout under an inert gas atmosphere.

In the first step a) the polyols and the diisocyanate are reacted in thepresence of 2,5,7,10-tetraoxaundecane and optionally a catalyst.

The reaction to obtain the isocyanate-terminated polyurethane prepolymermay be carried out in accordance with common polyurethane synthesis. Ina preferred embodiment, the first polyol and TOU are mixed untilhomogeneous, followed by the addition of the diisocyanate. If the firstpolyol is solid at room temperature, it is preferably melted beforeadding.

The second polyol may be added at any time. If it is solid at roomtemperature, it may be dissolved in TOU before addition or not. In apreferred embodiment the second polyol is DMPA and is added in solidstate, e.g. into a mixture of TOU, first polyol and diisocyanate.

Before addition of an optional catalyst, mixing of the components ispreferably carried out under stirring and heating. Stirring underelevated temperature for a certain time is usually suitable in order toobtain a homogenous mixture and optionally to reduce residual watercontained in the starting materials. Depending on the starting materialsadded, it may be suitable to continue stirring under elevatedtemperature for a certain time before adding the next starting material.

The mixing and stirring of the starting materials is preferably carriedout under an elevated temperature which depends on the startingmaterials used. A sui-table temperature during mixing and beforeaddition of a optionally present catalyst is e.g. in the range of 60 to100° C., preferably 70 to 90° C. The temperature during mixing may begradually raised.

If a catalyst is used, it is usually added as the last component. One ormore catalysts may be used. A huge range of catalysts suitable forpolyurethane synthesis may be used. The skilled person is familiar withthese catalysts. Examples of suitable catalysts are organotin compounds,tertiary amines, organobismuth compounds, organozirconium compounds,organoruthenium compounds, organotitanium compounds, organoironcompounds, organomolybdenum compounds and organozinc compounds.

Examples of organotin catalysts are dibutyl tin dilaurate,dibutylbis(laurylthio)stannate, dibutyltinbis(isooctylmercapto acetate)or dibutyltinbis(isooctylmaleate), or tin octaoate. Examples of tertiaryamines are DABCO, pentametyldipropylenetriamine, bis(dimethylamino ethylether), pentamethyldiethylenetriamine, DBU phenol salt,dimethylcyclohexylamine, 2,4,6-tris(N,N-dimethylaminomethyl)phenol(DMT-30), 1,3,5-tris(3-dimethylaminopropyl)hexahydro-striazine.

Further examples of catalysts are Bi(III) complex compounds or Zr(IV)complex compounds, particularly with ligands selected from alkoholates,carboxylates, 1,3-diketonates, oxinate, 1,3-ketoesterates and1,3-ketoamidates.

After addition of the optionally present catalyst, stirring underheating is continued. The polymerization may be carried out e.g. at atemperature in the range of 80 to 120° C., preferably 80 to 100° C. Theprogress of the reaction can be controlled by determining the NCOcontent in the mixture. If the desired or theoretical NCO content isreached, the reaction is complete and the mixture can optionally becooled down to a lower temperature, e.g. in the range of 20 to 80° C.The mixture contains an isocyanate-terminated prepolymer.

In the case, a second polyol having an acid group or an amine group isused to prepare the prepolymer, subsequently, a base, in case of polyolwith an acid group, or an acid, in case of polyol with an amine group,is added to the obtained isocyanate-terminated polyurethane prepolymerto convert the acid groups or amine groups, respectively, completely orpartially into ionic groups. The amount of the base or acid added ispreferably such that at least 50%, more preferably at least 80% andstill more preferably 90 to 100% of the acid or amine groups, preferablycarboxyl groups, of the isocyanate-terminated prepolymer are convertedinto ionic groups.

After neutralization, the prepolymer having ionic groups, in particularcarboxylate groups, is obtained. The neutralized isocyanate-terminatedpolyurethane prepolymer containing 2,5,7,10-tetraoxaundecane is nowready to be dispersed in water.

In the step c) the neutralized isocyanate-terminated polyurethaneprepolymer having ionic groups is dispersed in water and the chainextender, preferably a polyamine, is added. The amount of water used fordispersion is preferably such that the water content desired for thepolyurethane dispersion is achieved.

The temperature of the neutralized prepolymer in step c) is preferablyin the range of 15 to 100° C., more preferably 30 to 80° C.

The temperature of the water in step c) is preferably in the range of 2to 30° C., more preferably 5 to 25° C.

While or short after the precursor is dispersed in water, the chainextender is added e.g. in pure form or dissolved or dispersed in water.The chain extender reacts with the dispersed prepolymer, forming thechain-extended polyurethane. The chain-extender is preferably used insuch an amount, that its reactive groups, preferably primary orsecondary amine groups, are present in a ratio in the range of 0.8 to1.1, preferably 0.9 to 1, compared to the isocyanate groups.

The chain extension of the polyurethane prepolymer leads to an increaseof the molecular weight and the formation of a water-based polyurethanedispersion comprising a chain-extended polyurethane. The reactiveisocyanate groups react with the functional groups of the chain extendersubstantially more quickly than with water. After chain-extension, anyremaining free isocyanate groups are usually reacted with water underfurther chain extension.

After chain extension the water-based polyurethane dispersion comprisingthe chain-extended polyurethane and 2,5,7,10-tetraoxaundecane isobtained.

It is also possible to prepare the polyurethane dispersion by makingstep a) without 2,5,7,10-tetraoxaundecane and add2,5,7,10-tetraoxaundecane before or after step b) is made.

Another embodiment of the invention is a coating composition comprisingthe polyurethane dispersion described above and at least one furtheringredient selected from other polymer dispersions, fillers, pigments,other solvents, stabilizers, defoamers, surfactants, rheologicaladditives, thickening agents and antioxidants.

Other polymers dispersions are polymer dispersions different from thepolyurethane dispersion described herein. Other solvents are solventsdifferent from TOU.

Additionally, the coating composition may contain any further ingredientwhich is typically used in water-based coatings.

The coating composition or the inventive polyurethane dispersion can beused for coating any substrates. They are especially suitable forcoating a plastic substrate or a rubber substrate. The plastic or rubbersubstrate may be of any plastic type or rubber type known, for examplerigid or soft PVC, polycarbonate, polystyrene, polyester, PET, polyamid,PMMA, ABS, SAN, epoxy resin, phenolic resin, PUR, POM, TPO, PE, PP, EPMor EPDM, which can be surface treated, e.g. by plasma, corona or flame,or untreated. Preferred plastics are polycarbonate, polyester, PET,polypropylene, in particular biaxially orientated polypropylene or PVC,particularly rigid PVC.

The plastic or rubber substrate may be e.g. a plastic or rubber articleor an article having at least in part a plastic or rubber surface to becoated. The plastic or rubber substrate may be in the form of a plasticor rubber plate, a plastic or rubber sheet, a plastic or rubber object,e.g. a window frame, plastic or rubber fibres, a plastic or rubber cableor plastics or rubber woven into textiles.

The coating composition or the inventive polyurethane dispersion isparticularly useful for coating rigid PVC, such as window frames out ofrigid PVC.

The invention also relates to a process for coating a plastic substrateor a rubber substrate, comprising applying the coating composition orthe inventive polyurethane dispersion on a plastic or rubber substrateand hardening the applied coating or dispersion.

The application of the water-based polyurethane dispersion onto theplastic or rubber substrate may be carried out by any conventionalmethod known to the skilled person, e.g. by brushing, rolling, sprayingand industrial application equipment such as flexo or gravure printing.

The applied coating usually hardens by evaporation of water, i.e. byphysical drying. The remaining chain-extended polyurethane materialforms a film. This film forming process is supported by the solvent2,5,7,10-tetraoxaundecane, which helps the chain-extended polyurethaneparticles to coalesce and forming a macroscopically homogenous film. Itis also possible that there is a certain crosslinking taking placeduring or after the physical drying. But in most applications the curingof the coating is simply a physical process. Optionally, hardening canbe supported by heating, but this is usually not necessary.

The invention also relates to a coated article obtained by the processfor coating described above.

After application and drying, the final film shows excellent surfacequality and its adhesion on various substrates is very good.Particularly surprising is the good adhesion on plastic or rubbersubstrates. Further particularly surprising is the high ultimate tensilestrength of the dried dispersion.

Another embodiment of the invention is the use of2,5,7,10-tetraoxaundecane as coalescent for water-based polyurethanedispersions, particularly for rigid polyurethane dispersions, which havean elevated glass transition temperature (Tg) and which need acoalescent for film forming at ambient temperatures, particularly attemperatures below 30° C.

Such rigid polyurethane dispersions are typically based on polyesterpolyols and/or polycarbonate polyols, DMPA andbis(4-isocyanatocyclohexyl) methane, chain extended by a polyamine.

Surprisingly, 2,5,7,10-tetraoxaundecane acts as an efficient filmforming aid for rigid polyurethane dispersions. When such a dispersionor a composition containing such a dispersion comprising2,5,7,10-tetraoxaundecane as a coalescent for the polymer particles isapplied at ambient temperature in a thickness in the range of 10 micronsto 5 mm, preferably 50 microns to 3 mm, it dries by evaporation of waterand optionally other volatile ingredients and forms a rigid, elasticfilm.

Preferably, 2,5,7,10-tetraoxaundecane is used in an amount in the rangeof 3 to 60% by weight, preferably 10 to 40% by weight, based on thetotal weight of the chain-extended polyurethane.

Compared to conventional coalescents such as NMP or NEP,2,5,7,10-tetraoxaundecane adds additional benefits such as goodenvironmental, health and safety properties and improved adhesiveproperties, especially on plastic or rubber substrates. Particularlybeneficious is the excellent adhesion of polyurethane dispersionscontaining 2,5,7,10-tetraoxaundecane as coalescent on rigid PVC.

Additionally beneficial is the fact, that 2,5,7,10-tetraoxaundecane canalso be used in the preparation of the dispersion, helping to dissolveand dilute components during the production process.

EXAMPLES 1. Preparation of Polyurethane Dispersions: PUD-TOU-1:Isocyanate-Terminated Polyurethane Prepolymer:

27.58 parts by weight (pbw) of 2,5,7,10-tetraoxaundecane were placed ina round bottom flask under nitrogen atmosphere. Then 32.31 pbw of moltencopolycarbonatediol based on 3/1-mixture of 1,4-cyclohexane dimethanoland 1,6-hexanediol, OH-number 125 mg KOH/g were added under goodstirring and the mixture was heated to 60° C. Then 32.23 pbw Desmodur®W(bis(4-isocyanatocyclohexyl)methane, from Covestro) were added undergood stirring and the mixture was heated to 80° C. After one hour, 4.45pbw dimethylol propionic acid were added, followed by 0.06 pbw dibutyltindilaurate. The mixture was then heated to 90° C. After 3 hours, theisocyanate content of the mixture was 4.71 weight-%. The obtainedisocyanate-terminated polyurethane polymer was a clear liquid.

Neutralization:

The obtained isocyanate-terminated polyurethane polymer was cooled downto 70° C. and 3.36 pbw trimethylamine were added under good stirring.The neutralized isocyanate-terminated polyurethane prepolymer containingcarboxylate groups and 2,5,7,10-tetraoxaundecane was obtained, which isthe precursor for the polyurethane dispersion.

Dispersing and Chain-Extension:

42.65 weight parts (pbw) water were placed in a round bottom flask at atemperature of 25° C. Then 19.72 pbw of the neutralizedisocyanate-terminated polyurethane prepolymer described above, which hada temperature of 70° C., were added under very good stirring, while adispersion was formed. Then, 9.78 pbw ice was added under good stirringand another 19.72 pbw of the neutralized isocyanate-terminatedpolyurethane prepolymer described above, which had a temperature of 70°C., were added under good stirring to the mixture, followed by theaddition of 8.13 pbw of a solution of 30 weight-% Dytek®-A (2-methylpentamethylenediamine, from Invista) in water under very good stirring.The mixture was stirred for another 30 min. The obtained water-basedpolyurethane dispersion was a milky white fluid and containedapproximately 58.12% by weight water, 29.65% by weight chain-extendedpolyurethane and 10.88% by weight 2,5,7,10-tetraoxaundecane.

PUD-NEP-1: (Reference)

The same procedure as described for PUD-TOU-1 was repeated, except that2,5,7,10-tetraoxaundecane was replaced by the same amount ofN-ethyl-2-pyrrolidone.

2. Characterization of Polyurethane Dispersions:

The features of the dispersions are given in Table 1.

Viscosity is the dynamic viscosity determined at 20° C. (Brookfield,spindle 1, 50 rpm).

Solid content was determined by using an infrared-dryer from MettlerToledo. The ultimate tensile strength (UTS) and the elongation at break(EAB) were determined with a free film according to DIN EN 53504. Thefree film was prepared by pouring the dispersion into a polypropylenelid in a thickness of 2 to 3 mm (wet), which resulted in a dry filmthickness of approx. 1 mm. After 1 day at 23° C./50% rH, the dried filmwas removed from the lid and flipped over, so that the bottom side wasup, and let dry for another 28 days at 23° C./50% rH.

The freeze-thaw stability was determined by storing the dispersion in aclosed container for 24 h at −20° C. followed by 24 h at 23° C.,repeating this cycle three times. If the dispersion shows no change inaspect, it is judged as stable.

TABLE 1 PUD-TOU-1 PUD-NEP-1 (inventive) (reference) viscosity (20° C.)(Centipoise) 107 84 solid content (% by weight) 32.4 35.0 pH 10.7 10.5average particle size 30 nm 23 nm UTS (MPa) 25.2 6.2 EAB (%) 122 354freeze-thaw stability stable stable

3. Use as Coating:

Both dispersions were used as a coating.

Each dispersion was applied on an aluminium Q-panel from Q-Lab at a wetfilm thickness of 100 micron, resulting in a dry film thickness ofapprox. 30 to 32 micron. Each film was dried at 23° C./50% rH for 14days and used for measuring Pencil Hardness according to ASTM D3363,Persoz Hardness according to ASTM D4366 and adhesion according to ASTMD3359 (crosshatch).

Each dispersion was further applied on plastic substrates(polycarbonate, ABS, Nylon, rigid PVC (uPVC)), in the same way asdescribed for the aluminium Q-panel, and the adhesion was determinedaccording to ASTM D3359 (crosshatch).

The results are given in Table 2.

TABLE 2 PUD-TOU-1 PUD-NEP-1 (inventive) (reference) Pencil Hardness H HPersoz Hardness 283 247 adhesion on aluminium  7% Loss  45% Lossadhesion on polycarbonate  4% Loss 100% Loss adhesion on ABS 20% Loss100% Loss adhesion on Nylon 10% Loss  15% Loss adhesion on rigid PVC  4%Loss  70% Loss

1. A water-based polyurethane dispersion comprising a chain-extendedpolyurethane, wherein the polyurethane dispersion comprises2,5,7,10-tetraoxaundecane.
 2. The water-based polyurethane dispersionaccording to claim 1, wherein the chain-extended polyurethane has ionicgroups or contains polyethylene oxide units.
 3. The water-basedpolyurethane dispersion according to claim 1, comprising 3 to 60% byweight of 2,5,7,10-tetraoxaundecane based on the total weight of thechain-extended polyurethane in the water-based polyurethane dispersion.4. The water-based polyurethane dispersion according to claim 1,comprising 45 to 80% by weight of water.
 5. The water-based polyurethanedispersion according to claim 1, wherein the chain-extended polyurethaneis a reaction product of an isocyanate-terminated polyurethaneprepolymer and at least one polyamine as chain extender.
 6. Thewater-based polyurethane dispersion according to claim 5, wherein theisocyanate-terminated polyurethane prepolymer is a reaction product ofat least one first polyol, at least one second polyol, which isdifferent from the first polyol, and at least one diisocyanate, whereinthe at least one second polyol is selected from a polyether polyolhaving ethylene oxide units, a polyol having an acid group and a polyolhaving an amine group.
 7. The water-based polyurethane dispersionaccording to claim 1, wherein the water-based polyurethane dispersion isan anionic or non-ionic polyurethane dispersion.
 8. A process forproducing the water-based polyurethane dispersion according to claim 1,comprising the steps of a) reacting at least one first polyol, at leastone second polyol and at least one diisocyanate in the presence of2,5,7,10-tetraoxaundecane and optionally a catalyst to form anisocyanate-terminated polyurethane prepolymer, b) optionally at leastpartly neutralizing the isocyanate-terminated polyurethane prepolymer toobtain a neutralized isocyanate-terminated polyurethane prepolymer, andc) dispersing the optionally neutralized prepolymer in water and addinga chain extender to obtain the water-based polyurethane dispersioncomprising the chain-extended polyurethane and2,5,7,10-tetraoxaundecane.
 9. The process according to claim 8, whereinthe at least one second polyol has carboxyl groups and/or the at leastone diisocyanate is an aliphatic diisocyanate.
 10. The process accordingto claim 9, wherein the base to neutralize the carboxyl groups is atertiary amine.
 11. The water-based polyurethane dispersion according toclaim 1, obtainable by a process for producing the water-basedpolyurethane dispersion, comprising the steps of a) reacting at leastone first polyol, at least one second polyol and at least onediisocyanate in the presence of 2,5,7,10-tetraoxaundecane and optionallya catalyst to form an isocyanate-terminated polyurethane prepolymer, b)optionally at least partly neutralizing the isocyanate-terminatedpolyurethane prepolymer to obtain a neutralized isocyanate-terminatedpolyurethane prepolymer, and c) dispersing the optionally neutralizedprepolymer in water and adding a chain extender to obtain thewater-based polyurethane dispersion comprising the chain-extendedpolyurethane and 2,5,7,10-tetraoxaundecane.
 12. Coating compositioncomprising the water-based polyurethane dispersion according to claim 1at least one further ingredient selected from other polymer dispersions,fillers, pigments, other solvents, stabilizers, defoamers, surfactants,rheological additives, thickening agents and antioxidants.
 13. A methodcomprising applying the coating composition according to claim 12 or ofthe water-based polyurethane dispersion comprising a chain-extendedpolyurethane, wherein the polyurethane dispersion comprises2,5,7,10-tetraoxaundecane, for coating a plastic substrate or a rubbersubstrate.
 14. A process for coating a plastic substrate or a rubbersubstrate, comprising applying the coating composition according toclaim 12 or the water-based polyurethane dispersion comprising achain-extended polyurethane, wherein the polyurethane dispersioncomprises 2,5,7,10-tetraoxaundecane, on the substrate and hardening theapplied coating.
 15. Coated article obtained by the process according toclaim
 14. 16. A method comprising applying 2,5,7,10-tetraoxaundecane ascoalescent for water-based polyurethane dispersions.